The plasma-enhanced vapor may be used for deposition onto plastic articles, particularly for depositing a glass-like coating onto plastic bottles. The coating provides an enhanced gas-barrier and better adhesion compared with prior art coatings, and is suitable for pressurized containers, whose surface flexes and stretches, and whose internal pressure acts against an external coating. The primary component of the vapor is produced by evaporating, in an evaporative source, one or more solids and the deposition of the coating may be applied in conjunction with a reactive gas, or gases, to provide desired coating clarity or colorization. Further, it may be produced by using more than one evaporation source and solids of different boiling points.
Commercial applications of plastic articles have experienced a growth, because of the properties of these articles such as low-cost, light weight, flexibility, resistance to breakage, and ease of manufacture and shaping. However, plastics also have the disadvantage of relatively low abrasion-resistance and poor barrier properties against the permeation of vapors such as water, oxygen, and carbon dioxide. In food packaging applications, limitations in barrier properties have limited the use of plastics. For example, in the case of beverage bottles, inadequate barrier properties have restricted the use of smaller bottles required in some markets. Solutions to this problem, including the use of high-barrier plastics and coatings of various types, have been either uneconomical or have provided inadequate barrier-improvement or add expense to the known recycling processes.
A number of processes have been developed for the application of coatings on plastic, but these have been mainly for plastic films. Relatively few processes have been developed which allow the economic application of a glass-like coating onto preformed plastic containers such as PET bottles, where the demands on the coating's barrier performance are increased by the flexing of the walls of the bottle, the stretching of said walls under pressure, and the delaminating force due to the in-bottle pressure. Also, most processes are on the batch-production principle, and very few processes exist which can be applied to a continuously-running process.
U.S. patent application Ser. No. 08/818,342 filed by Plester et al on Mar. 14, 1997 allowed, and PCT International Application PCT/US98/05293 filed on Mar. 13, 1998 describe the use of an anodic arc for externally-coating beverage bottles and their disclosures are incorporated by reference herein in their entirety. Anodic arc systems are also described by Ehrich et al in U.S. Pat. Nos. 4,917,786; 5,096,558; and 5,662,741, the disclosures of which are also incorporated herein by reference.
The basic anodic system, as described by the prior art, has the following disadvantages:
a) The crucibles evaporative material content, such as silicon, cannot be replenished continuously when this evaporative material is in powder or pellet/chip form. PA1 b) The quantity of vapor evolved from the crucible depends partly on the degree of filling of the crucible with evaporative material. Since the degree of crucible-filling is a variable which constantly changes, this could present a control problem. PA1 c) The distribution, at various angular displacements, of the quantity of vapor evolved from the crucible, also depends partly on the degree of filling of the crucible with evaporative material. This makes it difficult to use the vapor from the crucible for the purpose of coating several articles simultaneously, without the risk that these will all receive different amounts of coating. PA1 d) The lips of the crucible are eroded by the anodic arc. This not only presents a maintenance problem, but it also means that the material of the crucible may thus be included in the coating composition and thereby reduce the performance of the coating. For example, crucibles for holding silicon are normally constructed of carbon, which is eroded and vaporized by the anodic arc and the carbon vapor is free to form a contaminant in the desired silicon or silicon dioxide coating. PA1 e) The said crucible lip erosion further affects the quantity of vapor evolved and the distribution of this vapor at various angular displacements around the crucible. PA1 f) Even where the crucible is independently heated (rather than intentionally heated by the anodic arc), the anodic arc represents a second and uncontrolled source of heating. This second source of heating partly affects the quantity of vapor evolved, irrespective of any control device for the crucible's independent heating system. This makes process control of evaporation rate difficult, whilst evaporation rate is an important parameter. PA1 g) The anodic arc energizes the plasma, but since an uncontrolled and unknown portion of this arc's energy is dissipated by evaporation of the material in the crucible, this makes the process control of the critical parameter of plasma-enhancement difficult. PA1 h) Since part of the energy of the anodic arc inadvertently causes evaporation, even in anodic arc systems with independent crucible heating, this limits the amount of energy available for plasma enhancement. PA1 i) Anodic arc systems employing independent crucible heating have complicated designs around the crucible in view of the conflicting needs, on the one hand to heat the crucible and on the other hand to provide a cooled anodic connection. This can result in additional cost and complication, oversized heating systems, and energy waste, as well as lead to crucible-damage on shut-down due to the cooling-effect of the anodic connection. PA1 j) Many applications, particularly those involving colored coatings, require the simultaneous evaporation of more than one solid substance. For barrier enhancement, it can also be desirable to add other substances to the base coating. Since such substances differ in boiling point, they cannot be combined in a single evaporating crucible, because evaporative fractionation within the crucible would lead to poor coating composition control. Therefore, multi-component coatings using the anodic arc system must be produced by a multi-series of anode-cathode couples, since one separate anodic arc source for each crucible is needed for process-control purposes. This not only makes a multi-component coating systems complicated and expensive, but also risks interference between the closely positioned array of anodic arcs. PA1 k) The cathode's evaporative material cannot be replenished continuously and it is therefore desirable in practice to use materials which erode slowly. This acts contrary to the desire to use the cathode for optimum plasma enhancement and ionization, since materials which achieve this often have a high erosion rate. The use of Zn, Cu, Al, noble metals, alkaline earths, and particularly Mg, has been found to be highly desirable, and in most cases continuous cathode replenishment is needed for economic operation. PA1 a) To enable replenishment, within the evaporator-crucible system, of the solid material to be evaporated and used for coating without interrupting the evaporator operation; PA1 b) To enable the said evaporator crucible to remain at substantially the same degree of filling during its operation; PA1 c) To provide a vapor particle distribution around the said crucible, which continuously remains constant and well directed; PA1 d) To provide an evaporation system where both the evaporator-energy supply to the crucible and the control of this energy are substantially independent of the energy supplied for plasma-enhancement; PA1 e) To provide an evaporation system with electric arc discharge plasma enhancement which has improved control of each system function, substantially avoids erosion or damage of the evaporator crucible, whose crucible can have a simpler design, which can operate with vapors whose deposited solids are non-conducting electrically, which enables several materials to be evaporated separately but enhanced by the same single arc; PA1 f) To enable continuous replenishment of the cathode's evaporative material; PA1 g) To enable high energy plasma through use of rapidly eroding materials at the cathode, particularly Mg, other alkali metals, and metals of relatively low boiling point; PA1 h) To enable materials produced by the erosion of the cathode (e.g. Mg, alkaline metals, low boiling point metals. etc.) to be incorporated as dopants in the coating; PA1 i) To enable substantially uninterrupted measurement and control in a continuously running coating process, of evaporation rate and degree of ionization; and PA1 j) To enable in-situ cleaning of vacuum enclosures without need to release vacuum, thereby enhancing the operation of continuously running coating processes.
Prior art exists (German Patent DE 4440521C1, Hinz et al) where the crucible is independently heated by electrical resistance or by thermal radiation, and where the anodic arc plasma-enhancement is provided separately by means of a cathode and a separate anode. However, the anode of such systems quickly becomes coated with the evaporated material from the crucible, or with plasma particles, or with the reaction product when a reactive gas is used. Such systems are therefore only usable where the coating is electrically conducting, since the anode would otherwise quickly become inoperative and the system would shut down. Since the barrier coating of plastic articles often requires the use of coatings with materials such as silicon, which are electrically non-conducting, such prior art cannot be used for many barrier coating systems.
It is important to control accurately the coating thickness on a plastic article and therefore highly desirable to be able to measure continuously, and in situ, the rate of deposition from an evaporative source, so that adjustments to the controls of the said evaporator source can be made as needed throughout the coating operation. Prior art provides means for measuring the rate of deposition by measuring the change of the oscillation frequency of a crystalline substance as the evaporated solids deposit on said crystalline substance. However, the crystalline substance quickly becomes coated and can no longer function, so the system is not usable for normal process control in continuous operating coating systems. A self-regenerating system for rate-of-deposition measurement is needed to enhance process control.
The quality of a coating on plastic articles, particularly the quality of the barrier property of coatings on plastic bottles, is dependent on he control of the degree of ionization and thus on the energy-level of the plasma. A suitably high-energy plasma enables the substrate surface to be cleared of dirt and inert molecules, promoting coating adhesion and coating purity, and further enables coating particles to become embedded in the substrate or to react with the substrate, additionally promoting adhesion. High-energy plasma also promotes the chemical reaction of coating particles with each other, thus forming a dense matrix on the substrate surface, which further enhances adhesion and barrier properties. Finally, high-energy plasma induces coating particles to be deposited in a flat, dense physical structure due to the impingement of high-energy collisions, enhancing coating continuity and denseness. On the other hand, over-energized plasma may overheat the substrate, or cause excessive decomposition or degassing from the substrate, or damage the coating. The evolution of gases from the substrate surface during its degassing mixes with the coating particles and reduces coating quality. It is thus important to measure and control plasma energy and degree of ionization. Prior art does not teach how this can be achieved.
An example of the need for controlled use of high-energy plasma is presented by barrier coating of plastic bottles for carbonated beverages. A barrier coating on a plastic bottle for carbonated beverages must desirably be able to flex, stretch, have adhesion capable of withstanding the pressure migration of the carbon dioxide from the inside of the bottle, and be robust and abrasion resistant in use. It is also desirably dense, preferably amorphous and continuous over the bottle surface. These properties rely on applying controlled. high-energy plasma.
All evaporator systems deposit particles within their enclosure, the latter being normally a high vacuum enclosure. Operation under vacuum is necessary so as to avoid heat damage of heat sensitive substrates such as plastic, and also to avoid gas phase reactions, which in turn would reduce the barrier and other qualities of the coating, since many of these desired properties rely on the on-surface interaction of the coating particles. Particles deposited within the vacuum enclosure tend to disturb the mechanical operation of the coating system and in particular tend to absorb volatiles and make vacuum pump-down more difficult. As a result, the walls of such vacuum enclosures must be cleaned regularly, and this involves production loss and shut-down. An in-situ cleaning system which enables regular and rapid cleaning of the enclosure internals without releasing vacuum and opening the enclosure is desirable for continuous operation and would improve economic operation by reducing downtime.